In a steelmaking and casting operation, batches of partially refined molten steel are tapped from a basic-oxygen or electric-arc furnace into a refractory-lined ladle. Final refining to the specified chemical composition is performed in the ladle which is then drained into a bathtub-type vessel, also refractory lined, called a tundish, that simultaneously drains into water-cooled copper molds where the steel solidifies into a specific shape such as slab, bloom or billet.
The initial transfer from furnace to ladle is executed by tilting the furnace and draining the liquid contents through an opening in the furnace shell known as a taphole. After refining, and with the ladle upright, the transfer of liquid steel to the tundish is controlled by a slide-gate valve attached to a refractory nozzle in the ladle bottom. Likewise the draining rate from the tundish is controlled by one or more slide-gate/nozzle combinations in the tundish floor, or by a vertically movable refractory plug over the nozzle known as a stopper.
The cast steel is pulled continuously into a cooling bed by pinch rolls underneath the mold. While in motion, the hot slabs, blooms or billets issuing from the casting machine are cut to length prior to further rolling.
Typically, a string of ladles of refined steel is drained sequentially into the same tundish before changing tundishes by an operation known as a tundish "fly."
An inevitable consequence of the furnace-ladle-tundish-mold transfer operations is the presence of a slag layer over the molten steel. In the steelmaking furnace, a significant amount of "oxidized" slag is generated that is detrimental to final refining of the steel to the targeted composition. Thus, in the transfer of steel from furnace to ladle through the taphole, it is desirable to prevent significant carryover of furnace slag. In ladle refining, the objective is to form a "reducing" slag that is prepared by deliberate addition of appropriate fluxing agents. Although this reducing slag is not deleterious to the refined steel from the standpoint of chemical reactivity, carryover into the tundish in the form of entrained droplets, if not completely de-entrained before the steel solidifies, compromises the surface and internal quality of the cast product. Furthermore, some slag is eventually generated in the tundish itself by melting of; (1) "free-opener" sand, and; (2) insulating powder added to the tundish to form a protective blanket over the liquid surface. If a significant amount of liquid slag inadvertently reaches the mold, the rate of heat extraction by the mold is diminished, creating an opportunity for the liquid core in the solidified steel shell to break out, a highly unwelcome event that disrupts operations, causes equipment damage and carries the risk of a life-threatening explosion.
In the initial transfer of liquid steel from a furnace to a ladle, technology has been developed to limit the amount of furnace slag carried over into the ladle. For example, in an electric-arc furnace equipped with an eccentric-bottom-tapping (EBT) system, virtually slagfree tapping is achieved by the geometric configuration of the taphole relative to the furnace hearth and by melting surplus steel scrap that is retained in the furnace as a liquid reserve, known as a heel, after filling the ladle to the desired weight. However, in the event the scrap charge is "short," there is a risk of significant carryover of oxidized slag into the ladle. This slag must be removed by skimming, an operation affecting overall productivity, yield, and electrical energy consumption. Thus, in order to maximize the benefit of the EBT configuration, a slag-detecting system is required that shuts off the liquid flow automatically before the amount of slag in the ladle becomes significant. Effective slag-detecting devices installed near the taphole are available, but no art has been developed for measuring the amount of slag retained in the furnace. Knowledge of the amount of slag retained in the furnace has significant value, particularly in the production of low-phosphorus steel.
Effective slag-detecting devices have also been developed that limit the amount of ladle slag carried over into the tundish. However, such devices rarely achieve an operational availability of 100 percent. If, for example, the slag alarm fails in only one of ten ladles drained into a particular tundish, the amount of slag present before the tundish is removed from service is subject to a serious degree of uncertainty, undermining the effectiveness of tundish weight measurements (determined by load cells) to gauge the depth of the steel bath. To be certain that slag does not reach the mold when an unknown amount is present in the tundish, the tundish is shut off early, incurring a yield penalty in the form of a larger-than-necessary tundish "skull." In addition, the lining-wear profiles of individual tundishes vary over time, amplifying the lack of precision in the relationship between tundish weight and steel bath depth. Furthermore, as mentioned above, casting operations are susceptible to adverse events if the steel bath depth happens to stray outside safe limits.
Various systems have been developed for measuring the liquid level in remote storage vessels such as water tanks. Some examples are shown in U.S. Pat. No. 3,461,722 to Martens and U.S. Pat. No. 4,903,530 to Hull. One method is based on a change in the magnitude of an electrical current flowing in a circuit when an insulated electrode, placed at a known elevation inside the tank, makes or breaks contact with the liquid surface. The electrical circuit requires a power source, which typically supplies a constant DC voltage, allowing circuit resistance to be measured directly. Since the electrical circuit is open when the electrode is not in contact with the liquid, the change in resistance between an open and closed condition is massive, typically several orders of magnitude.
In baths of molten metal, particularly molten steel, measurement of liquid level is complicated by the presence of a supernatant slag layer of unknown thickness. U.S. Pat. No. 4,365,788 to Block; U.S. Pat. No. 3,395,908 to Woodcock; U.S. Pat. No. 3,505,062 to Woodcock; U.S. Pat. No. 4,413,810 to Tenberg; and U.S. Pat. No. 3,663,204 to Jungwirth teach that a change in the resistance of a sensing circuit can be utilized to detect the steel-slag interface level. However, the theoretical basis and application of such a resistance measuring device is suspect, for example, if the containment vessel for the liquid steel has an internal diameter at the slag-steel interface of 3 meters and contains an extraordinarily thick layer of molten slag of 0.5 meter, the resistance of such a layer, top to bottom, is approximately 0.00007 to 0.002 ohm, far below the threshold needed for reliable interpretation. As a practical matter, the minimum length of copper-conductor cable required to deliver the sensing circuit signal to a signal converter or control pulpit a safe distance away from the hot vessel, is approximately 20 to 30 meters. The resistance of a single strand of 16 gage copper wire, 20 meters long, is approximately 0.5 ohm at ambient temperature. Despite molten slag having a specific resistivity about 7000 times greater than liquid steel, which in turn has a specific resistivity about 70 times higher than ambient copper, such differences are overwhelmed by the relatively large volume of, and short conducting distances in, the liquid phases such that these methods are not easily and economically usable.
U.S. Pat. No. 4,365,788 to Block also teaches that combination electrodes embedded in the wall of a metallurgical vessel can be used to measure variables such as lining wear, liquid level, steel-slag interface level and temperature, based on changes in resistance of a circuit with an applied power source. However, Block does not teach how a change in the single parameter of resistance can distinguish between multiple phenomenological causes. Furthermore, internally generated DC voltages at junctions between dissimilar conductors at high temperature, such as caused by Seebeck and double-layer effects, make circuit resistance changes difficult, if not impossible, to interpret.
U.S. Pat. Nos. 4,037,761, 4,150,974, and 4,235,423, all to Kemlo, disclose interface level detection based on a change in voltage output from a single conducting probe. Three designs are disclosed, one for measurement of slag-layer thickness and interface level in a full ladle, and two for interface level detection during ladle draining. For the measurement in a full ladle, a moveable electrode is disclosed that is suspended above the ladle and that can be made to travel vertically through the slag layer by means of a winching device. The moveable electrode assembly comprises a conducting metal rod or tube connected by a non-conducting black-rubber, or ebonite, bushing to a second tube. A conductor wire from a suitable instrument passes through an opening in the upper tube through the bushing to the lower rod where it terminates.
However, such a device cannot provide a reliable indication of slag-layer thickness because of the extreme hostility of the environment above a ladle of molten steel. It is well known that a piece of metal immersed in molten slag would immediately become coated with frozen slag that would then take some time to melt off. Once the electrode is in contact with molten metal the only way of knowing whether the tip barely protrudes through the slag layer or is several inches below it, is to withdraw the probe. Since a solid conductor in contact with liquid metal would start to dissolve and/or melt, capture of the exact elevation of the slag-metal interface is problematical. In addition, as a practical matter, the slag surface could be solidified into a hard crust that requires a rugged "push-pull" mechanism and a second, sacrificial probe to penetrate.
For the interface level measurement during ladle emptying, one embodiment is a conducting probe embedded in a refractory sleeve of a stopper rod employed for draining the ladle. With the advent of effective slide-gate valves, the use of stopper rods to control steel flow from ladles has virtually disappeared. In an alternative embodiment a single electrode of steel, graphite or molybdenum is embedded in a lining brick located near the centerline of the ladle trunnions to detect when the steel-slag interface passes this elevation as the ladle is drained. However, regardless of the selection of electrode material, this design is susceptible to two performance-impairing phenomena. The first is caused by the electrode having a higher thermal conductivity than the refractory brick in which it is embedded, known as electrode "fogging," where a skin or "skull" of solidified metal forms over the electrode, effectively extending it downwards by an unknown distance. The second factor is electrode "blinding," that arises from the widespread practice of extending ladle lining life by a patching practice known as "gunning," in which a jet of refractory slurry is directed at high wear areas of the ladle lining. Here the objective of always maintaining contact between the electrode and molten steel is at loggerheads with the objective of maximum time interval between ladle relines.
Some additional examples of interface level detection are disclosed in U.S. Pat. No. 4,345,746 to Schleimer, U.S. Pat. No. 5,827,474 to Usher et al. U.S. Pat. No. 5,375,816 to Ryan; U.S. Pat. Nos. 5,549,280 and 5,650,117 to Kings et al disclose methods for detecting the presence of a non-metallic liquid phase in a flowing steel stream and interpreting the properties of the sensing-circuit signal.
Thus, there remains a need for an economical and practical device that determines the location of the molten metal-slag interface and slag-air interface in containment vessels that overcomes the limitations of the prior art and provides a reliable signal for halting a liquid transfer operation at the optimum moment in time. Furthermore, there is a need for a device that can determine the location of the molten metal-slag interface in a tundish so that inadvertent discharge of slag or overflow of steel can be prevented.